Moored on the waters of San Francisco Bay, just east of Alcatraz and the Golden Gate Bridge, is -- well, a mystery.

Atop a barge docked at Treasure Island, site of a former Navy base, is a four-story structure made from shipping containers. The purpose of the structure remains unknown, but the safe money says Google is up to something on board.

Officials won't say exactly what's going on there. But some sleuthing by the media in the tech-heavy Bay Area has pretty much nailed down that Google, headquartered about 40 miles south in Mountain View, California, is behind the project.

Tech blog CNET reported as much on Friday, citing lease agreements, Treasure Island locals and "tracking a contact tied to the project on LinkedIn."

News reports say an unknown company has been working on a major project, under tight security, in a hangar on Treasure Island since last year. Witnesses say materials from that hangar were transferred onto the barge.

Google did not respond to a request to comment for this story.

So, even if the suspicions about Google and the barge's mysterious cargo are correct, that leaves the obvious question: What are they building in there?

Early speculation has centered on a data center.

Google has banks of servers stored in warehouses all over the world, and floating data centers aren't unheard of. Sitting on that much water would provide an obvious source of cooling, which is a big concern for data centers, and possibly even a source of power. And Google has a patent for such a project.

But San Francisco station KPIX, a CNN affiliate, threw a twist into the narrative, quoting unnamed "sources close to the project" as saying the structure will be retail space -- a sort of floating Google store. Google hopes to tow the structure across the bay to San Francisco, where it would be used to market Google Glass, the company's connected eyewear, KPIX reported.

Google Glass, the company's big leap into wearable technology, puts computer capabilities into eyeglasses. The headsets so far have been released only to early testers and are expected to go on sale to consumers in the coming months.

An official with the San Francisco Bay Conservation and Development Commission told KPIX that there have been discussions with Google about "hypothetical operations" on the bay.

But work on the project suddenly stopped a few weeks ago. One possible reason: An insider close to the commission told KPIX that Google "can't park this barge on the waterfront without a permit, and they don't have one."